Metrology and the Updated SI

Metrology and the Updated SI: Episode I - A New Day

On May 19, 2019, the International System of Units (SI) was changed in a fundamental way so that all the units have their basis in seven defining constants. This was nothing short of colossal, and enabled our familiar, beloved, and world-wide system of units to be more accurate, available and reproducible than ever before. Though few would disagree with the motivations that were behind the redefinition, many metrologists struggle with the practical implications that the redefinition presents in terms of performing measurements that are “traceable to the SI.” In this first of a series of webinar “episodes,” we would like to introduce and explore the redefinition of the SI in 2019 from both theoretical and practical viewpoints. Any coherent system of units is only as good as the measurements that it produces, so it is vital that a metrology practitioner understands exactly how the SI revision affects her own realization and dissemination of the SI. This is a broad topic, but we hope to provide useful information on the changes to all the SI quantities: mass, thermodynamic temperature, length, electrical current, amount of substance, luminous intensity, and time. In subsequent episodes, we plan to give a more detailed discussion of changes to mass, temperature, and electrical current (along with voltage and resistance), as these quantities are most affected by the recent revision.

An interesting result of the redefinition is that the units of voltage, resistance, and electrical current will no longer have the conventional (non-SI) definitions that were adopted in 1990. The Ampere, the unit of electrical current, is now defined by the electron charge and not by a theoretically impossible experiment involving infinitely long wires of negligible cross-sectional area. The units of voltage and resistance are no longer anchored to the 1990 values of the Josephson and Von Klitzing constants, respectively, as these have now been fixed for all time with no uncertainty under the 2019 revision of the SI. The net result is that these electrical units are once again within the SI; hence, there is no longer any need for the potentially confusing conversions between the conventional units of 1990 and the SI.

Metrology and the Updated SI: Episode III - The IPK Strikes Back!

By now everyone knows that for the first time in 130 years, the definition of the kilogram was changed to something other than the mass of the International Prototype Kilogram (IPK) artifact. The magnitude of the unit of mass is now such that the Planck constant has a fixed value of 6.62607015×10−34 J∙s with no uncertainty. Ultimately, this definition will ensure greater accessibility, reproducibility, and scalability of mass, but that is not yet the case. Because of larger-than-anticipated dispersion among the Planck constant measurements of some National Metrology Institutes (NMIs), the world has agreed to a gradual phase-in of the Planck-based mass realization over the next several years. Until this dispersion has been appropriately reduced, the IPK will continue to serve as the world’s realization for mass (albeit with a first-ever uncertainty), giving way eventually to a world consensus kilogram value prior to allowing individual mass realizations by NMIs.

About Pat Abbott
Patrick is a staff member at NIST where he has been involved with measurement services for nearly 28 years. He joined NIST in 1992 as a member of the Pressure and Vacuum Group and was responsible for the dissemination of the unit of pressure in the range of 10-1 Pa to 10-7 Pa. In 2007, Patrick joined the Mass and Force Group to work on the NIST “Mise en Pratique” (practical method) for realization and dissemination of the revised kilogram definition that became part of the revised SI in May of 2019. Along the way, Patrick has acquired a working knowledge of the 2019 SI revision and has taught a course in resulting changes for mass metrology for the past several years. Patrick has represented NIST internationally at meetings of the Consultative Committee for Mass and Related Quantities, and currently is the Mass Calibration Project Leader within the Mass & Force Group. He holds B.S. and M.S. degrees in Physics.